Page 355 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH11A 5/30/2001 4:49 PM Page 312

312 Applied Process Design for Chemical and Petrochemical Plants

Purge Two classes for refrigerants are “A” and “B” and can be
identified as follows:
The purge device or system removes noncondensables
from the system at a minimum loss of refrigerant. “Class A signifies refrigerants for which toxicity has not
been identified at concentrations less than or equal to
Process Performance 400 ppm, based on data used to determine Threshold
Limit Value-Time Weighted Average (TLV-TWA) or
Refrigerants
consistent indices” from Section 6.1.2. 20,27
Many materials are suitable for refrigerant purposes, and “Class B signifies refrigerant for which there is
each usually has some special characteristics that allow it to evidence of toxicity at concentrations below 400 ppm,
serve a particular application better than some of the others. based on data used to determine TLV-TWA on
Before selecting a refrigerant, it is important to evaluate its consistent indices” from Section 6.1.2. 20,27
flammability and toxicity data, pressure-temperature-volume
relationships, enthalpy, density, molecular weight, boiling In identifying toxicity, its class is followed by a number
and freezing points, and various effects on gaskets, metals, designating flammability. The identifying numbers are
oils, etc. 16
• One (1) for refrigerants with no flame propagation
ANSI/ASHRAE Standard 34-1992, “Number Designation potential.
and Safety Classification of Refrigerants” • Two (2) for refrigerants with low flame propagation
potential.
The two purposes of ASHRAE Standard 34-1992 are • Three (3) for refrigerants with high flame propagation
potential.
1. “ . . . to establish a simple means of referring to
common refrigerants. It also establishes a uniform A summary from ASHRAE Standard 34-1992, prepared by
27
system for assigning reference numbers and safety The Trane Co., is used with permission of ASHRAE 34-
classifications to refrigerants.” [from Section 1] 20, 27 1992, American Society of Heating, Refrigerating, and Air
2. “This standard provides an unambiguous system for Conditioning Engineers, Inc., ©1992:
numbering refrigerants and assigning composition-
designating prefixes for refrigerants. Safety classi- “Class 1 indicates refrigerants that do not slow flame
fications based on toxicity and flammability data are propagation when tested in air at 101 kPa (14.7 psi)
included.” [from Section 2] 20, 27 and 18° C (65°F).
“Class 2 signifies refrigerants having a lower flamma-
3
The ASHRAE Standard cited here (with addenda bility limit (LFL) of more than 0.10 kg/m (0.00625
3
through 1994) provides refrigerant Safety Group Classifica- lb/ft ) at 21°C and 101 kPa (70°F and 14.7 psia) and a
tions related to toxicity and flammability. Tables 11-3A, heat of combustion of less than 19,000 kJ/kg (8,174
11-3B, and 11-3C are the official ASHRAE Refrigerant Data Btu/lb). The heat of combustion shall be calculated
and Safety Classifications of all refrigerants used. Note that assuming that combustion products are in the gas
all the latest replacement/new refrigerants are not included phase and in their most stable state (e.g., C, N, S give
in the 1992 addenda through 1994, because addenda have CO 2 , N 2 , SO 3 ; F and CI give HF and HCL if there is
been issued through 1995 as of this writing. Most of the enough H in the molecule, otherwise they give F 2 and
newest and replacement refrigerants are presented in man- CI 2 ; excess H is converted to H 2 O).
ufacturer’s data to follow. ANSI/ASHRAE Standard 15-1994, “Class 3 indicates refrigerants that are highly
“Safety Code for Mechanical Refrigeration,” responds to the flammable, as defined by an LFL of less than or equal
3
3
rapid development of new refrigerants and refrigerant mix- to 0.10 kg/m (0.00625 lb/ft ) at 21° C and 101 kPa
tures for use in new and existing equipment. 20 (70°F and 14.7 psia) or a heat of combustion greater
®
Toxicity is referenced to the “Threshold Limit Value — than or equal to 19,000 kJ/kg (8,174 Btu/lb). The heat
Time Weighted Average” established for each refrigerant. of combustion is calculated as explained above in the
20
This is defined in ASHRAE Standard 15-1994 as (refer to definition of a Class 2 category.” from Section 6.1.3
the manufacturer’s product data for more complete detail):
ANSI/ASHRAE Standard 15-1994, “Safety Code for
“the refrigerant concentration in air for a normal Mechanical Refrigeration,” should be studied, examined,
8-hour work day and a 40-hour work week, to which and complied with by the design engineer.
repeated exposure, day-after-day, will cause an adverse
effect in most persons” from Section 3. 20,27 (Text continues on page 317)